What kind of rattling noise are we talking about here. I think mine makes a relatively faint rattling pretty much until the idle RPM drops on cold starts. Do I have a cam phaser issue? Some have described a clunk, which I'm definitely not hearing. Also not something I can hear from inside the truck.
Lots of info on this, but basically stand in front of the truck and start it with your remote in the morning. If you hear a loud rattle for a few seconds while the oil pressure comes up, that's likely the phasers. The waste gates can rattle too, but it's not nearly as loud. Bad part is the locking pins in the phasers which is supposed to hold them in place on a cold start, but they break somehow.
Mine has 50k on it, 2018. It's on the list to get the phasers done in a few weeks (waiting on parts). Service writer, who is a great guy and a Raptor owner, said they also order other parts for the cams so that if they find those to be bad, they'll replace them at the same time. Ford is allowing them to get everything it could possibly need, so it doesn't have to be torn down twice. If it doesn't need the other parts, they don't use them and send them back.
I think the bottom line from all the posts I've read is you need to find a dealer with a service writer that isn't a moron. Oz at Lakewood Ford in the Denver area is awesome, for anyone in the area. I was shocked when I called two other dealers a lot closer to my house, and they seemed less than interested in even looking at it. Not sure why, I'd think it'd be a good Ford paid job for any service dept. Who knows.